The present invention relates to methods and apparatus for treating a waste liquid with a gas and more particularly to methods and apparatus for oxygenating wastewater as, for example, in an activated sludge secondary stage waste treatment process.
In order to avoid intolerable ecological and environmental damage, efforts have continually been made to provide wastewater treatment plants of sufficient capacity to adequately treat or purify increasing quantities of municipal sewage and industrial wastes. One well known technique for purifying wastewater is the activated sludge process. In this process, aerobic microbial activity is relied upon to cause a digestion or consumption of organic waste materials. In order to sustain such microbial action, oxygen must be dissolved into the wastewater (which preferably comprises a mixed liquor of liquid waste and microbial solids) at rates proportional to the microbial activity. The rate of oxygen dissolution required generally varies in accordance with the biological oxygen demand (BOD) of the particular wastewater being treated and the microbial population. A number of techniques are currently used or proposed for obtaining the required dissolution rates.
Briefly, prior art techniques for oxygenating wastewater include an air aeration process wherein ambient air is relied upon as an oxygen source and by vigorously agitating the wastewater, such as by means of a surface impeller, a transfer of oxygen into the wastewater is achieved. However, as the oxygen content of air is only 21 percent, this dissolution technique requires substantial wastewater `retention` time and relatively high expenditures of electrical power in driving such mechanical mixing devices. In other prior art techniques, oxygen enriched air has been proposed as a means of reducing the aforementioned retention time and in this regard, systems for diffusing minute bubbles of oxygen into wastewater have been suggested. Although capital costs of such systems are relatively low, it has been found that the amount of oxygen which is not dissolved in the wastewater, and hence is released to the atmosphere is many times of an economically unacceptable level. This wastage of oxygen is particularly noticeable in relatively shallow secondary treatment tanks. Other techniques for oxygenating wastewater include down-flow bubble contact devices wherein an oxygen enriched gas is injected into a rapidly descending current of wastewater. The momentum interchange between such fluids tends to reduce the rise velocity of the gas bubbles, thereby increasing the period of time during which oxygen and wastewater are maintained in contact and thus enabling a transfer of oxygen into the wastewater. This technique, as for example described in U.S. Pat. Nos. 3,476,366 and 3,643,403, has the disadvantage that substantially complete dissolution of oxygen into the wastewater cannot be assured and that any oxygen not so dissolved is generally lost to the atmosphere.
Still further prior art attempts to efficiently oxygenate wastewater include the concept of covering a secondary stage treatment tank and providing an oxygen enriched atmosphere in the ullage space so formed above the wastewater therein. Such a technique is described in an article entitled "Aeration With a High-Oxygen Atmosphere in A. S. Process," by Harold E. Babbitt, Wastes Engineering, May, 1952, pages 258-259. In this system a feed gas of approximately 95 percent oxygen was bubbled through wastewater to be treated with undissolved oxygen being collected in the ullage space, recompressed and recirculated as a feed gas to the closed aeration tank. Similar techniques wherein oxygen is bubbled through wastewater and/or mixed with the wastewater by means of a submerged turbine or surface impeller are described in U.S. Pat. Nos. 3,547,811-3,547,815. It is recognized, however, that both the Babbitt system and the latter techniques require confining a relatively large volume of an oxygen-enriched aeration gas in the ullage space beneath a tank cover and as such, provide a substantial supply of a combustion supporting medium. As rotating mechanical mixing equipment as described in the foregoing patents provides a potential source of sparks and combustible material exists in the form of lubricants, potentially dangerous fire hazards are presented.
As each of the foregoing prior art techniques for treating wastewater possesses certain undesirable characteristics it is clear that a present need exists for improved methods and apparatus for dissolution of oxygen in wastewater by an efficient mass transfer action as well as safely effecting a reduction of the power requirements necessary for such mass transfer and tank mixing.